This invention relates to improvements in a process for the production of hydrogenated nitrite rubber.
Hydrogenated nitrite butadiene rubber (HNBR) is a valuable elastomer known for its combination of unique properties, including high tensile strength, resistance to abrasion, high oil resistance and resistance to oxidation. HNBR may be produced by the homogeneously catalyzed selective hydrogenation of NBR with hydrogen in an organic solvent. In the context of the invention, xe2x80x9cselective hydrogenationxe2x80x9d is understood to mean the hydrogenation of the olefinic carbon-carbon double bonds, with the carbon-nitrogen triple bonds remaining intact. The expression xe2x80x9cwith the carbon-nitrogen triple bonds remaining intactxe2x80x9d in this context means that less than 7%, preferably less than 5%, more preferably less than 3% and, most preferably, less than 1.5% of the nitrite groups originally present in the NBR are hydrogenated. The hydrogenation may be monitored by IR or NMR spectroscopy.
Rhodium and ruthenium compounds are commonly used to catalyze such hydrogenations (see, for example DE-PS 25 39 132, DE-OS 33 37 294, 34 33 392, 35 29 252, 35 40 918 and 35 41 689, EP-A 134 023 and 298 386 and U.S. Pat. Nos. 3,700,637, 4,464,515, 4,503,196 and 4,795,788).
A preferred catalyst has the formula:
(RmB)lRhXn
in which each R is, independently, a C1-C8-alkyl group, a C4-C8-cycloalkyl group a C6-C15-aryl group or a C7-C15-aralkyl, B is phosphorus, arsenic, sulfur, or a sulphoxide group S=0, X is hydrogen or an anion, preferably a halide and more preferably a chloride or bromide ion, I is 2, 3 or 4, m is 2 or 3 and n is 1, 2 or 3, preferably 1 or 3. Preferred catalysts are tris-(triphenylphosphine)-rhodium(I)-chloride, tris(triphenyl-phosphine)-rhodium (III)-chloride and tris-(dimethylsulphoxide)-rhodium(III) -chloride, and tetrakis-(triphenylphosphine)-rhodium hydride of formula ((C6H5)3P)4RhH, and the corresponding compounds in which triphenylphosphine moieties are replaced by tricyclohexylphosphine moieties. The catalyst can be used in small quantities. An amount in the range of 0.01 to 1.0% preferably 0.02% to 0.6%, most preferably 0.06% to 0.12% by weight based on the weight of polymer is suitable.
The hydrogenation reaction can be carried out in solution. The solvent must be one which will dissolve nitrile butadiene rubber, in which the limitation excludes the use of unsubstituted aliphatic hydrocarbons. Suitable organic solvents are aromatic compounds including halogenated aryl compounds of 6 to 12 carbon atoms. The preferred halogen is chlorine and the preferred solvent is a chlorobenzene, especially monochlorobenzene. Other solvents that can be used include toluene, halogenated aliphatic compounds, especially chlorinated aliphatic compounds, ketones such as methyl ethyl ketone and methyl isobutyl ketone, tetrahydrofuran and dimethylformamide. The concentration of polymer in the solvent is not particularly critical but is suitably in the range from 1 to 30% by weight, preferably from 2.5 to 20% by weight, more preferably 6 to 15% by weight and most preferably 10 to 15% by weight. The concentration of the solution may depend upon the molecular weight of the copolymer rubber that is to be hydrogenated. Rubbers of higher molecular weight are more difficult to dissolve, and so are used at lower concentration.
It has recently been observed that, when using the aforementioned rhodium catalysts for the hydrogenation of NBR in monochlorobenzene solvent, there is a significant amount of corrosion in the plant equipment in which this process is carried out. The appearance of such corrosion is unexpected, as there is no obvious causative agent present which could initiate the corrosion process. This corrosion is a problem because it leads to extensive damage of plant equipment and necessitates costly repair work, resulting in production down-time. Further, such corrosion may result in product contamination which, obviously, seriously affects product quality. Thus, the presence of corrosion has a serious effect on overall productivity.
It has been determined that the aforementioned appearance of corrosion in the plant equipment in which hydrogenated nitrile rubber is produced is due to the presence of relatively large amounts of HCl. Since there is no obvious source of this acid in the reaction mixture, its appearance was entirely unexpected. It has been determined that the generation of HCl is, uniquely, a result of the particular conditions under which the hydrogenation of NBR is carried out. The HCl is actually generated by the hydro-dehalogenation of the monochlorobenzene solvent under the reaction conditions.
The addition of a compatible weakly basic additive, such as Epoxidized Soy Bean Oil (ESBO), to the reaction mixture has alleviated this serious problem.